1. Field of the Invention
The invention relates to apparatus for scarfing, more particularly for scarfing an inspection track on a flat surface of a steel slab, particularly a slab of ductile steel of deep drawing quality, to enable inspection of the slab. The invention also relates to a method of scarfing.
2. Description of the Prior Art
Scarfing processes are carried out on steel for three distinct reasons. The first is to remove flaws such as cracks in a piece of steel, e.g. a slab. This is carried out by means of a hand-held flame torch, which is used to burn away the metal entirely in order to remove the flaw. The operator applies the torch and moves it according to the nature of the flaw being removed. Secondly, scarfing is used to remove a complete surface layer of the steel, particularly with a view to removing any flaws which may be present at the surface. One apparatus designed for carrying out this second process is described in GB 847,993, in this case, the entire surface of a steel bar is removed by passing the bar through a scarfing apparatus which provides an annular inwardly directed flame. The bar is moved past the apparatus. Guide means for inserting the bar are provided, and also for guiding the bar after the burned-away surface portion has been removed.
The third purpose for which scarfing is used is to inspect the quality of a steel slab. When a slab is rolled into sheet, flaws which may not be visible on the surface of the slab are revealed in the sheet. It is therefore desirable to estimate the number of such subsurface flaws in the slab, before it is rolled. This may be done, as described in more detail below, by scarfing an inspection track on the slab.
Although the apparatus according to the invention might be applied to the removal of flaws, it is particularly designed and intended for quality inspection of a steel slab. The invention will be explained here in relation to this purpose.
A criterion for the quality of a slab of steel which is to be rolled is thus the number and type of defects which lie 1 to 3 mm deep under the surface of the slab. These defects, which are formed by inclusions of Al.sub.2 O.sub.3 and SiO.sub.2 and, in the case of continuously cast slab, also by residual casting powder, come to the surface during the further working of the slab due to oxidation of the sheet surface during heating in a furnace and hot-rolling. The oxidized surface layer falls off exposing the steel beneath. They determine the quality of the surface in the finished product. This surface quality is in many cases of great importance, for instance in the case of car bodywork.
A known method for judging the quality of a slab of rolled steel consists of scarfing an inspection channel on the slab surface using a hand-held blow-torch, the channel having a depth equal to the thickness of the layer which will subsequently be lost by oxidation. The number and type of defects in the inspection channel can then be ascertained. By inspection channel (or track) we mean here a channel obtained by scarfing over a width of e.g. about 10 cm for at least a part of the length of the slab parallel to its longitudinal direction.
A problem in this conventional method of scarfing an inspection track by hand is that accuracy of the estimation of quality is dependent on the competence of the operator in forming an uninterrupted channel of constant depth with a blow-torch. The channel tends to be irregular because the operator, when forming the channel, moves continuously along the slab, so that the spacing of the burner of the blow-torch and the slab surface varies. The channel depth is therefore not constant.